<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="JavaScript/jquery.min.js"></script>
</head>
<style>
    body {
        height: 2000px;
    }
    
    .back {
        position: fixed;
        width: 50px;
        height: 50px;
        background-color: pink;
        right: 30px;
        bottom: 100px;
        display: none;
    }
    
    .container {
        width: 900px;
        height: 500px;
        background-color: skyblue;
        margin: 400px auto;
    }
</style>

<body>
    <div class="back">返回顶部</div>
    <div class="container"></div>
    <script>
        $(function() {
            $(document).scrollTop(100);
            var boxTop = $(".container").offset().top;
            // 页面滚动事件
            $(window).scroll(function() {
                if ($(document).scrollTop() >= boxTop) {
                    $(".back").fadeIn();
                } else {
                    $(".back").fadeOut();
                }
            });

            // 返回顶部
            $(".back").click(function() {
                // animate做的是元素的动画而不是文档的动画
                $("body, html").stop().animate({
                    scrollTop: 0
                });
            })
        })
    </script>
</body>

</html>